Structural features of large molecular mass material in coal-derived liquids: catalytic hydrocracking of the pyridine-insoluble fraction of a coal-tar pitch

The pyridine-insoluble fraction of a coal-tar pitch has been catalytically hydrocracked, The starting sample contained no material that could be observed by gas chromatography/mass spectrometry (GC/MS) or by heated-probe mass spectrometry, The aim of the study was to generate structural information on this narrow cut of large-molecular-mass material, which consisted mainly of compounds boiling above 450 degreesC, A much broader boiling-point distribution was found for hydrocracked products; evidence from all analytical techniques used (size-exclusion chromatography, W-fluorescence spectroscopy and heated-probe mass spectrometry) indicated a significant reduction of the molecular mass range. A wide variety of aromatic groups was identified in hydrocracked products by heated-probe mass spectrometry, ranging from phenanthrene (m/z 178) to beyond dibenzocoronene (m/z 400), Ions corresponding to alkyl (m/z 43, 57, 71 and 85) and alkenyl groups (m/z 41, 55, 69 and 83) were detected. The results reflect the ability of the hydrocracking process to cleave bonds within large pitch molecules, releasing the structural units which form the molecules. The structure of the original pyridine-insoluble material may thus be seen as a series of aromatic groups linked by short bridges or small groups, but with some longer aliphatic bridges (C-12 and C-17) as shown by pyrolysis-GC/MS, Clearly, some molecular species were reduced in mass but still lay above the range of detection by heated-probe mass spectrometry and could not be identified. Although the extent to which large polycyclic aromatic entities themselves have been cracked (or otherwise reduced to smaller polycyclic aromatic groups) cannot be quantitatively discerned from the present data, the hydrocracking of isolated fractions of intractable coal-derived material appears to offer a useful method for probing their structural features.